The field of the invention is serial printers and the invention relates more specifically to assemblies for holding and controlling the movement of the print head.
With the increased use of computers and word processors, there has been an increased interest in devices which convert the electronic data to a permanent image on paper. Teletype machines and electric typewriters have given way to daisy wheel, dot matrix and ink jet printers. While such serial printers are capable of far higher print speeds than the earlier types of printers, there is always a desire to further increase print speeds. Other types of print heads have been developed and will continue to be developed which also require that the print head be driven along the paper at speeds which are higher than those possible with the present devices. Furthermore, the printer manufacturing business has become very competitive and there is a need to reduce production cost to stay competitive.
For all serial printing methods, it is essential that the print head be moved parallel to the surface of the platen against which the paper or other printing substrate is held. It is therefore common for the print head to be mounted on a carriage which is moved by a guide means along the platen. Typically the carriage is held by two bearings which ride on two ground steel rods which are mounted on the frame of the printer. Typically, the mounting position of these rods is adjustable so that the final unit may provide an exact parallel movement of the print head with respect to the platen. Such adjustment, of course, is time consuming and the ground steel rods are themselves expensive. A commonly used bearing surrounds the steel rod and is fabricated from a polymer having a low coefficient of friction against the ground steel rod. A commonly used second bearing is another block of polymer having a U-shaped groove therein which is pressed against the second steel rod. Another commonly used bearing is a bronze bushing riding on a steel rod. The inherent rubbing friction of these carriage bearing surfaces would limit the speed at which the printer may be operated. Rubbing friction creates heat which tends to expand the mating members generating more friction. This condition generally requires larger print head drive motors to overcome the friction at higher speed. Therefore, the print speed for such units must be limited to that which does not generate sufficient heat to create this problem. The steel rods are also relatively heavy and the increased weight increases shipping, packaging and handling costs.
In addition to cost and print speed limitations, the standard ground steel rod mounting system is expensive both to manufacture and to assemble. Still further, the friction between the bearings and the rods requires a larger than necessary drive motor. There is thus a need for an improved apparatus for mounting the print head.
One such improved system is shown in U.S. Pat. No. 4,201,423 assigned to the assignee of the present application. This device discloses two or more precision, linear, ball-bearing slides held in special end-alignment support caps and the carriage is held by the inner slide member of the two or more slides. This system, although useful for some applications, is not capable of being used for all applications because of the space required to mount the two or more slides and the end alignment support caps. Furthermore, a lower cost system is required for some applications.